Digitalis glycosides in serum, urine, and cerebrospinal fluid, determined with a commercial radioimmunoassay.

We evaluated a commercial (Clinical Assays, Inc.) 125I radioimmunoassay for digitoxin for use in measuring cardiac glycosides in serum, buffer, urine, and cerebrospinal-fluid samples. We also assessed interference of seven cardioactive metabolites of digitoxin, including digoxin, with the assay. The precision of the assay was adequate for all matrixes studied for glycoside concentrations less than 5 microgram/L. Reactivity in the assay was dependent on the nature of the matrix: buffer and serum samples containing digitoxin showed a stronger reactivity than did samples of urine and cerebrospinal fluid. Patients' samples must therefore be analyzed with a standard curve prepared by use of the same matrix. The digitoxosides of digitoxin showed a slightly higher reactivity in the assay than did digitoxin itself, but digitoxigenin showed a 50% lesser reactivity. Digoxin, its digitoxosides, and digoxigenin hardly react at all in the assay and so do not interfere with digitoxin determinations.